New notification provider: CallMeBot API

[BothimTV]

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Description

This adds the support for CallMeBot.com in order to have WhatsApp notifications.
This additionally allows you to use it for Signal, Telegram Message + Call and Facebook Messanger. I didn't explicitly added the support for Signal and Telegram (as Message) because this was already implemented. But this still works as this is just a simple GET request to the user generated endpoint.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

Event	Before	After
UP	-
DOWN	-
Certificate-expiry	-
Testing	-

[CommanderStorm]

Overall, this looks pretty god to me.
I have left two inline comments

[CommanderStorm]

Thanks for the new notification provder!

All changes in this PR are small and uncontroversial
⇒ merging with junior maintainer approval